This invention relates to a start assist device for use with an electric motor having low starting torque, and particularly to a device for transferring angular momentum from a motor driven element to an element coupled to a load. More particularly, the invention relates to a device which provides substantially instantaneous transfer of momentum from the motor driven element to the element coupled to the load by means of an impact between respective surfaces of the elements.
Some electric motors, and particularly many types of single phase induction motors, develop inherently low starting torque, as compared to the running, or load torque. In some applications, unusually high starting torque requirements may exist continuously by design, or intermittently due to variations in operating conditions or mechanical malfunctions. In the latter case, unanticipated torque demands may cause a motor to be unable to start.
Conventional dishwashers typically rely on a sliding seal (e.g., carbon-ceramic surfaces) in the bottom of the dishwasher housing where the drive shaft penetrates the wall. Prolonged periods during which a dishwasher is not used may result in "sticking" of this seal due to contamination by detergents or water or other corrosion processes. The "stuck" seal represents an additional frictional barrier to starting the dishwasher that a low torque motor is sometimes unable to overcome, resulting in the need for a service call or other repair efforts. Thus, a need exists for a device which will provide an effective increase in the starting torque characteristic of a motor, such as a single phase induction motor of the type used to power an appliance such as a dishwasher, so as to allow the motor to start and function normally in the face of unanticipated high starting torque demands.
Start assist devices for use with electric motors are known. See, for example, U.S. Pat. No. 3,521,097 to Trinter; U.S. Pat. No. 3,153,159 to Lord; U.S. Pat. No. 4,305,304 to Jessup et al.; and U.S. Pat. No. 4,709,586 to Moutardier.
Conventional start assist devices generally consist of one or more springs coupled to a driving member and a driven member. The driving member is accelerated by the motor, thereby applying tension to the spring. In reacting to the applied tension, the spring tends to drag the driven member up to speed, providing for increased starting torque through a gradual acceleration of the driven member. The spring absorbs some of the energy meant to be imparted to the driven member to bring it up to speed, with a resultant decrease in the overall efficiency of the device. Furthermore, the gradual transfer of energy from the driving member to the driven member may not be adequate or appropriate in all situations, including the "stuck" seal situation referred to above.
A device that provides a substantially instantaneous transfer of energy from the driving member to the driven member would overcome these liabilities. Utilizing impact for the transfer of energy would take advantage of the relatively high running speed and associated angular momentum of the driving member to provide a substantially instantaneous transfer of energy to the driven member for improved starting assist. At the same time, use of an impact start assist device would avoid the use of springs and thereby provide an improved starting assist without the energy losses and other problems inherent in the arrangements which use springs. Thus, a device that provides for a transfer of energy from a driving member to a driven member by utilizing impact would represent an improvement over conventional start assist devices. An object of the present invention is to provide such an impact start assist device for use with electric motors.
According to the present invention, an impact start assist device for use in an electric motor comprises a driven member coupled to a load and a drive member coupled to the motor and to the driven member. The driven member and the drive member are rotatable, relative to each other, through at least a portion of a revolution. The device further comprises means for providing a substantially instantaneous transfer of angular momentum from the drive member to the driven member after relative rotation of the members through at least said portion of a revolution.
In one embodiment, the impact start assist device further comprises means for locking the drive member and the driven member together at high rotational speeds. Means is also provided for unlocking the drive member and the driven member at low rotational speeds, and at rest.
Illustratively, the means for providing an instantaneous transfer of angular momentum includes at least one anvil surface formed on the driven member and at least one hammer surface formed on the drive member. The hammer surface is positioned to impact the anvil formed on the driven member after the drive member has rotated through at least a portion of a revolution relative to the driven member.
The locking means illustratively includes at least one sprag that moves from a rest position to a wedged position between the hammer surface and the anvil surface at a first relatively high rotational speed. The locking means further includes a resilient element, such as a spring, coupled to the sprag to move the sprag from the wedged position to a rest position at a second relatively low rotational speed. The device also includes a resilient element coupled to the drive member and the driven member to return the members to predetermined angular positions relative to each other at a third relatively low rotational speed.
The impact start assist device of the present invention may be constructed as a coupling device which is used to connect a standard electric motor to a load. Alternatively, the impact start assist device may be integrally constructed with an electric motor, or the rotor of an electric motor may be constructed of two, relatively rotatable components which comprise driving and driven members and which function to provide the substantially instantaneous transfer of momentum in the manner described.
By providing a drive member which is initially free of the load, the present invention allows the drive member to quickly develop a high angular momentum. By rotatably coupling the drive member to the driven member and providing for a substantially instantaneous transfer of the drive member momentum to the driven member, the present invention provides a considerable improvement over conventional start assist devices.
Additional objects, features, and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of preferred embodiments exemplifying the best mode of carrying out the invention as previously perceived.